The Whitehall source claimed Mrs May’s move to relax her professed “red lines” in the European Union negotiations will backfire and result in a “seismic moment” for her party.

“If that narrative takes hold, 1997 will look like a vicar’s tea party,” the source told The Telegraph.

Speaking on Saturday, former Conservative leader Iain Duncan Smith, accused the Prime Minister of betrayal, suggesting the agreement will essentially be a “continued membership” of the EU’s customs union and single market, for goods and agricultural products.

Mr Duncan Smith said: “If the public perceive that not to be delivered, then the Government, I’m afraid, will suffer the consequences at the next election.”